Migrate away from the newly deprecated GlNav and GlNavItem

UI Polish

Problem

We're deprecating <GlNav> and its child components in GitLab UI. Currently this is used in https://gitlab.com/gitlab-org/customers-gitlab-com/-/commits/main/app/frontend/javascripts/navigation/components/nav_bar_items.vue and we'll need to migrate this to use <GlDropdown> or similar.

Currently used in the header bar

Proposal

Migrate the usage away from <GlNav> and towards <GlDropdown>. If you need help with this, the groupdesign system team will be on-hand.

GlDisclosureDropdown could be a possible candidate to switch for.